In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced.
It is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge.
It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared (IR) region. A monitoring or gauge apparatus for this purpose is commonly employed. Such an apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head, which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as, required by the individual machines. The gauges typically use a broadband infrared source such as a quartz tungsten halogen lamp and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a the case of a scanning gauge, are scanned in synchronism across it, and the scatter type (typically called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web. While it is most common to position IR moisture gauges in the more benign dry-end environment, similar gauges are also employed in the hostile wet-end of the papermaking machine. The wet-end moisture gauges are typically located at the end of the press section or the beginning of the dryer section. Gauges in these locations are useful for diagnosis of press and forming sections of the paper machine, or for “setting up” the web for entry into the dryer section.
U.S. Pat. No. 7,291,856 to Haran et al. describes a moisture sensor that uses high brightness superluminescent light emitting diodes (SLEDs) in conjunction with fiber optic delivery to achieve small and compact moisture measurements in hostile and space restricted environments. Specifically, the moisture sensor, which generates non-dispersive spectroscopic measurements of water in paper, is configured so that the sensitive opto-electronic and opto-mechanical components are positioned away from the hostile environment. At the same time, the sensor is capable of delivering a sufficient level of optical power to the measurement location that enables the sensor to maintain measurement speed and repeatability. One drawback of this technique is its limited coarse spectral resolution and limited wavelength range which ultimately restricted its application to measuring moisture. Moreover, in the case of monitoring moisture in paper, the limited spectral diversity of the light source yields data that is grade specific. As a result of this grade dependency, an elaborate calibration procedure is required in order to accommodate papermaking machines that produce a range of weight grades or paper that contains different components, e.g., paper additives.
The industry is in need of a versatile sensor that is capable of measuring a number of different parameters including moisture, temperature and cellulose fiber content early in papermaking processes. Such a sensor will enable better control of the process thereby minimizing off-specification product and minimizing paper breaks.